The art of printing involves reproducing an image by repeatedly transferring ink from an object bearing a master image to the substrate being printed upon, such as paper. There are many different ink transfer processes used in printing to transfer the ink from the image to the substrate, including relief, planographic, gravure (or intaglio), screen and electrostatic.
The relief process of printing involves forming a printing plate bearing the master image by relieving those portions of the plate that will not transfer ink. Thus, the non-relieved areas are those raised portions of the plate that will retain ink when the plate is pressed against a surface coated with ink. The printing plate is subsequently brought into contact with the paper and the inked, raised areas will transfer the image to the paper.
One form of relief printing is flexography. In flexographic printing, a flexible printing plate bearing the master image is mounted on the surface of a printing plate cylinder. In operation, the printing plate cylinder rotates so that the printing plate is brought into rolling contact with the substrate to be printed and will print one image for each revolution of the plate cylinder. An ink transfer roll is also positioned in rolling contact with the printing plate on the plate cylinder and resupplies ink to printing plate after it has printed an image on the substrate.
Ink transfer rolls are formed with a textured surface of small pits or cells that continuously pick up ink from an ink reservoir and apply that ink to the printing plate surface. The ink metering cells increase the volume and enhance the uniformity of the layer of ink transferred by the ink transfer roll.
Ink transfer rolls are conventionally formed of steel. Steel rolls are heavy, and their weight can render it difficult for the operator to intermittently change rolls as the printing operation may require, and the weight of the rolls can result in personal injury to the printing press operator. In addition, the weight of the steel rolls will occasionally cause the press operator to strike the rolls against hard objects such as the floor or the press itself and chip the edges of the printing surface, often rendering the roll unusable. Steel rolls are also subject to corrosion that can pit the ink transfer surface or bearing journals.
Conventional steel rolls are often coated with a ceramic surface layer, such as flame sprayed chromium oxide, that provides a hard, wear resistant rolling surface. The chromium oxide layer can be mechanically engraved to form the ink transfer cells. Alternatively, the chromium oxide layer can be engraved with a laser engraving machine that directs bursts of light energy onto the surface to form the desired pattern of cells. Laser engraving allows custom engraving with precise control over cell geometry, volume and alignment.
Ink transfer rolls made of steel are typically manufactured from standard hollow steel tubing to minimize material costs and unnecessary weight. Headers are inserted at either end of the tube and include journals that seat the roll on journal bearings in the printing press.
A close dimensional tolerance is required in ink transfer rolls to ensure that an even coating of ink is applied to the printing plate and that the rolls are dynamically balanced at high speeds. More particularly, a dynamically imbalanced roll can "bounce" when rotated at relatively high speed, and such bouncing can result in poor print quality since the roll is only intermittently in proper contact with the printing plate. Standard steel tubing, however, has relatively loose tolerances for roundness of the outside diameter and for concentricity of the outside diameter with the inside diameter. Thus, the outside diameter of a conventional ink transfer roll must be machined before it is engraved to obtain the necessary degree of roundness. In addition, weights usually must be positioned within the roll to correct for the unbalance caused by the lack of concentricity with the inside diameter.
An aluminum ink metering roller for use in lithographic printing has been proposed by Hycner et al. in U.S. Pat. No. 4,862,799. The '799 patent discloses a hollow aluminum base roll whereon ink transfer cells are formed by mechanical or diamond-stylus engraving. The engraved aluminum roll is then anodized to form a hard and wear resistant aluminum oxide surface layer, and this surface layer is coated with a thin and relatively soft copper layer to give the roll the necessary hydrophobic and oleophilic qualities required by lithographic printing.
As noted above, the ink transfer rolls used in flexographic printing are most commonly made from hollow steel tubing. Rolls made from aluminum or other lightweight materials have not been generally used since such rolls are not as strong as a corresponding steel roll and may allow undesirable flexing of the roll, particularly when used in a flexographic process. For example, an insufficiently stiff roll can be deflected or bowed by the hydraulic pressure of the ink being squeezed at the nip, and the resulting bowing of the roll can also create uneven inking of the printing plate and thus poor print quality.
It is accordingly an object of the present invention to provide a lightweight ink transfer roll adapted for use in a flexographic printing operation that overcomes the above noted deficiencies of the prior art rolls.
It is a further object of the present invention to provide a lightweight ink transfer roll of the described type which is easy to manually handle by press operators, thereby providing for quicker and easier press set up and clean up, reduced operator injuries, and reduced incidence of accidental edge chipping and damage to the roll.
It is a more particular object of the present invention to provide a lightweight ink transfer roll of the described type which, by reason of its light weight, possesses low dynamic inertia and low dynamic imbalance, thereby permitting the roll to run smoothly and to minimize bounce and vibration in the system which can lead to poor print quality.
It is another object of the present invention to provide lightweight ink transfer roll of the described type wherein the roll has sufficient stiffness to avoid bowing resulting from its contact with the printing plate and the hydraulic pressure at the nip.
It is still another object of the present invention to provide an efficient method of manufacturing an ink transfer roll having the described advantages and properties, and wherein the method produces a dynamically balanced product without the need for a post manufacturing balancing operation.